The xylenes, para-xylene, meta-xylene and ortho-xylene, are important intermediates which find wide and varied application in chemical syntheses. Para-xylene upon oxidation yields terephthalic acid which is used in the manufacture of synthetic textile fibers and resins. Meta-xylene is used in the manufacture of plasticizers, azo dyes, wood preservers, etc. Ortho-xylene is feedstock for phthalic anhydride production.
Xylene isomers from catalytic reforming or other sources generally do not match demand proportions as chemical intermediates, and further comprise ethylbenzene which is difficult to separate or to convert. Para-xylene in particular is a major chemical intermediate. Adjustment of isomer ratio to demand can be effected by combining xylene-isomer recovery, such as adsorption for para-xylene recovery, with isomerization to yield an additional quantity of the desired isomer. Isomerization converts a non-equilibrium mixture of the xylene isomers which is lean in the desired xylene isomer to a mixture which approaches equilibrium concentrations.
In general, a xylene production facility can have various types of processing reactions. One is a transalkylation in which benzene and/or toluene are reacted with C9+ aromatics to form more methylated aromatics. Another is xylene isomerization, which may also include ethylbenzene conversion, where a non-equilibrium mixture of xylenes is isomerized. The ethylbenzene may be isomerized to xylenes or may be dealkylated to yield, e.g., benzene. And another is disproportionation in which toluene is disproportionated. The disproportionation reaction yields one mole of benzene per mole of xylene produced.
The production of xylenes is practiced commercially in large-scale facilities and is highly competitive. Concerns exist not only about the effective conversion of feedstock to product xylenes, but also other competitive aspects with respect to such facilities including capital costs and energy costs. A prior art aromatics complex flow scheme has been disclosed by Meyers in part 2 of the Handbook of Petroleum Refining Processes, 2d. Edition, in 1997 published by McGraw-Hill.
In addition to improvements in catalysts for various of the reactions that may be used in the process such as isomerization, transalkylation and disproportionation, efforts have been expended to develop process flow schemes for at least one of reducing operating costs, improving conversion of the feedstock to sought product or reducing capital costs.
U.S. Pat. No. 4,341,914 to Berger discloses a transalkylation process with recycle of C10 alkylaromatics in order to increase yield of xylenes from the process. The transalkylation process is also preferably integrated with a para-xylene separation zone and a xylene isomerization zone operated as a continuous loop receiving mixed xylenes form the transalkylation zone feedstock and effluent fractionation zones. See also, U.S. Pat. No. 6,512,154.
U.S. Pat. No. 6,740,788 to Mahar, et al., discloses a process in which the feed to a transalkylation reactor is fractionated in a benzene column prior to being passed to the reactor.
U.S. Pat. No. 6,774,273 to Xie, et al., discloses a process for producing xylenes containing a transalkylation section, a disproportionation section and an isomerization section.
U.S. Pat. No. 6,867,339 of Kong, et al., discloses a process for producing xylenes containing a transalkylation section, a disproportionation section and an isomerization section.
US 2004/0186332 of Kong, et al., discloses a process for producing xylenes using a disproportionation and transalkylation of toluene and heavy aromatics.
Still a need exists to improve the economics of xylene production facilities.